1. Field of the Invention
This invention relates generally to apparatus for reproducing video signals recorded on a magnetic tape and, more particularly, is directed to apparatus for reproducing video signals recorded on a magnetic tape of the type having a manual track searching arrangement.
2. Description of the Prior Art
Recent developments with magnetic video tape recorders (VTR) of he helical scan type have enabled the substitution for broadcast purposes of such VTRs for conventional VTRs of the 4-head type. For example, such helical scan type VTRs which use a 1-inch format have even been standardized as type C by the Society of Motion Picture and Television Engineers (SMPTE) in the United States.
One of the reasons for such substitution and an advantage with helicals can type VTRs is that the video signals are recorded in parallel tracks extending obliquely on a magnetic tape with one field of video information being recorded in each track. Consequently, the helical scan type VTR is adapted to reproduce the video signals while operating in various special modes, such as still or stop mode, slow motion mode, fast-forward mode and a reverse slow motion mode. Since the magnetic tape used in the helical scan type VTRs is transported or moved longitudinally between supply and take-up reels, such special modes can be obtained merely by controlling the speed and direction at which the tape is transported between the reels and also controlling the rotation of rotary magnetic heads so that such heads are rotated at the frame frequency. In order for the rotary magnetic heads to accurately scan the parallel tracks at the different speeds, VTRs of the helical scan type have recently been developed in which the rotary magnetic heads are mounted on a deflectable element, such as a piezo-ceramic plate, which is controlled by a control signal based on the reproduced signal for deflecting the position of the heads to accurately scan the parallel tracks on the tape. In this manner, the parallel tracks are accurately scanned in the special reproducing modes so as to obtain a reproduced picture any guard band noise.
Because of such special reproducing modes, the helical scan type VTR is particularly adapted for searching a specific picture or frame from one of the tracks, for example, during an editing operation. In order to perform such searching function, there has recently been developed a helical scan type VTR with a manual operating wheel for controlling the speed and direction of movement of the magnetic tape in dependence on the speed and direction, respectively, of rotational movement of the operating wheel. Such helical scan type VTR was developed by the assignee of this application and is shown more particularly in U.S. Pat. No. 4,161,001, which issued on July 10, 1979. With the apparatus described therein, movement of the magnetic tape can be controlled by the manual operating wheel in a so-called "jog mode" which includes combinations of the special reproducing modes, such as the forward and reverse slow motion modes and the still reproducing mode, so that a particular frame can be located.
It should be appreciated that the accuracy of reproduction of a particular picture will depend, at least in part, by the amount of tension on the tape wrapped about the guide drum of the helical scan type VTR. If the amount of tension on the tape does not correspond to a desired condition, the reproduced output from the rotary magnetic heads do not accurately correspond to the picture recorded on the tape. Even further, in an extreme case, picture deterioration may result in portions of the reproduced picture being dropped entirely. The control of tension on the tape extending about the guide drum thus becomes critical, particularly in the jog mode of operation, for example, when the tape is moved with the following motions, stop.fwdarw.slow motion, and reverse slow motion.fwdarw.stop.fwdarw.forward slow motion. This is because the tension on the tape during such transitional movements also changes and must therefore correspond to desired tensions for the different movements of the tape.